A base station of a wireless communication system transmits a signal having an average power which is largely different from a peak power. In recent years, it is considered to employ a Doherty amplifier in order to enhance efficiency of a transmitting amplifier which is used in a transmitter of the base station, etc.
In the Doherty amplifier, a signal distributor distributes an input signal into two amplifiers. A carrier amplifier always amplifies one signal, and a peak amplifier amplifies only signal with a predetermined higher power level in the other signal.
Besides, in the Doherty amplifier, an output of the carrier amplifier is combined with an output of the peak amplifier through a Doherty combiner, and an impedance transformer is placed between the power combining point and a load (e.g. refer to Patent Literature 1 and Non Patent Literature 1). The impedance transformer is placed in order to mutually transform characteristic impedance between Z0 and (½)×Z0.
The Doherty combiner and the impedance transformer are basically composed of a quarter wavelength transmission line.
Besides, each of the characteristic impedance Z0 of the Doherty combiner and the impedance transformer is determined on the basis of desired impedance transformation ratio depending on a configuration of the Doherty amplifier. In the technology described in Patent Literature 1, for example, the characteristic impedance Z0 of the Doherty combiner is set at 50Ω and the characteristic impedance Z0 of the impedance transformer is set at 35.4Ω (=(50×25)^(½)).